The most ancient eukaryotes were unicellular organisms that probably resembled certain members of the Protista. The evolutionary origins of eukaryotes as well as the identification of specific protist groups that gave rise to the Fungii, Plantae, and Animalia remain obscure. An accurate history of eukaryotic microbial evolution is needed but efforts to construct consistent phylogenetic relationships for protists are frustrated by their enormous cytological and biochemical diversity. During the past two years, quantitative molecular phylogeny studies have revolutionized our views of eukaryotic evolution. Comparisons of ribosomal RNA sequences show that eukaryotes represent an ancient lineage that may be as old as the archaebacterial and eubacterial lines of descent, and the "higher" eukaryotic divisions arose nearly concurrently during a period characterized by a massive diversification of forms. Given the limited number of organisms that have been included in our studies, it is not yet possible to delineate ancestral relationships that gave rise to the fungal, plant, and animal kingdoms nor is it likely that the full extent of eukaryotic diversity is represented in our phylogenetic tree constructions. The goal of this proposal is to continue our analysis of genetic diversity within the eukaryotic kingdom by comparing complete small subunit ribosomal RNA gene sequences from a variety of protozoans, and, hence, to construct a consistent quantitative phylogenetic tree from the Eukaryota.